The Old Trafford manager has praised the young South American for his performances this season, suggesting he could become the greatest right-back to have played for the club
The 22-year-old has been an ever-present in the Premier League for United this season, and his early cross eventually led to Robin van Persie's goal in the 2-1 win over Arsenal on Saturday.
The Old Trafford manager believes the Brazilian can go on to enjoy similar levels of success to Gary Neville at the club - and suggested the youngster could become even better than the former United captain.
Asked by the club's official website whether Rafael could be as good as Neville, Sir Alex replied: "Definitely, there's no doubt about that. Rafael is definitely the more naturally talented.
"He's a Brazilian who trains like a beast. He never misses training - the boy is just a football fanatic. When he goes home in the summer, he plays every day for his street team back in Brazil."
Although Rafael showed signs of naivety earlier in his career - namely the red card against Bayern Munich in the 2010 Champions League quarter-final which United went on to lose - Sir Alex believes the player has turned a corner.
"Maybe he had rashness and the impetuosity of a young boy but somewhere along the line that maturity comes along and the rashness disappears. His form this season has been brilliant and the boy is really tough now," said the Scot.
He then drew a parallel between Rafael and Neville: "Gary was not the finished article when he came into the team but was lucky in the sense that he came in at a time when we were trying to push forward all our young players.
"But Gary improved and became absolutely fantastic - probably the best right-back the club has ever had."